Bacterial canker of stone fruit is caused by the two related bacterial species, Pseudomonas syringae and P. morsprunorum. The disease can occur on all stone fruit and is known as gummosis, blossom blast, spur and twig blight, sour sap, and dieback. The bacteria have a wide host range, can systemically invade host tissue without symptoms, and can grow epiphytically on host and nonhost leaf surfaces. As a result, control of this pathogen is complicated.

Symptoms

The disease occurs on branches, twigs, buds, leaves, and fruit. The most conspicuous symptoms are the cankers that exude gum during late spring and summer. Gumming is common on stone fruit trees, whether on trunks, limbs, twigs, or fruit when injuries occur. Thus, the name gummosis does not define a cause, only a response. Cankers on the twigs are darkened areas often at the base of buds. On limbs or trunks they are often darker than the normal bark, sunken in their centers, and they may extend for a considerable distance. Leaves and shoot growth beyond the canker may wilt and die during the growing season. Sour sap often occurs following winter injury to the trunks or limbs. The sour odor is due primarily to the fermentation of sugars by yeasts under the bark of injured areas. So this term, too, does not describe a cause. Gumming and sour sap, however, are symptoms of bacterial canker infection.

During periods of cool, wet weather after bloom, leaf and fruit infections may be common. Infected leaf and flower buds are killed during the dormant season. Small cankers often develop at the base of these dead buds. Occasionally infected dormant buds open normally in the spring only to wilt and die shortly after. In contrast, leaves and flowers from other infected buds may remain symptomless. Leaf infections, especially on cherry, appear as water-soaked spots that become brown and dry. Sometimes they are surrounded by yellow halos. Infected leaves may abscise during midseason. Lesions on green cherry fruit are brown and surrounded by water-soaked tissue.

Disease Cycle

The bacteria overwinter in the margins of cankers, in buds, and systemically within the tree. In the spring, during wet periods, the bacteria multiply and ooze from the cankers. They are spread by rains and enter the plant through natural openings or wounds. Periods of frequent rains, cool temperatures, and high winds are most favorable for infection.

Prolonged periods of cold, wet frosty weather in late spring or when storms occur that injure blossoms and leaf tissue are conditions often associated with outbreaks of bacterial spot. Freezing predisposes tissue to infection. When temperatures begin increasing in late spring and summer, disease development stops. At this time, the newly formed buds become infected either through leaf scars or bud scales.

Disease Management

Bacterial canker is much more severe on cold-injured trees and on trees growing in sites with poor soil drainage. Therefore, practices that minimize wounding and freeze injury and maximize good cultural practices will reduce the incidence of bacterial canker. The bacteria can be transmitted by pruning tools so these should be disinfected between trees if bacterial canker is present.

Cankers should be pruned several inches below obvious infection during the winter when temperatures are freezing or below. The choice of cultivar is important, for some stone fruit are more susceptible than others to bacterial canker. Sprays during the growing season have not been effective in disease control. Chemical control of bacterial canker is based on protective copper sprays. These sprays, however, protect from initial infection but cannot prevent the canker phase once infection has begun. Also, there is reported resistance to copper in orchards that have consistently used copper. Stone fruit crops are sensitive to copper and injury is common.